The rapid expansion of the wireless communications industry has increased the demand for frequency spectrum such that operators must be sure to use the spectrum as efficiently as possible. Innovative digital modulation and compression techniques, as well as spatial techniques such as cellularization and sectoring can be used to increase spectral efficiency.
Digital modulation is currently the most common method of transmitting information. It is more reliable and more spectrally efficient than its predecessor, analog modulation.
There are several popular techniques of digital modulation. Binary Phase Shift Keying (BPSK) is a simple method wherein binary characters "0" and "1" are each represented by two different phases of a carrier frequency in a channel. By simply alternating between these two phases, a transmitter can convey digital information to the recipient. The repeated variations create a signal that occupies a finite bandwidth. This bandwidth can be calculated and is directly related to the data rate and the modulation scheme. The spectral efficiency of a modulation scheme is measured in "bits per hertz," a ratio that should be maximized. BPSK has an ideal spectral efficiency of 1 bit per hertz. Quadrature phase shift keying (QPSK), using four phases, is a more efficient modulation scheme, and has an ideal spectral efficiency of 2 bits per hertz.
As more data is transmitted through a given channel size, the condition of the channel becomes more important. A noisy channel can prevent the receiver's rf modem from recovering the data. Each type of modulation scheme has a certain tolerance to unwanted signals. This tolerance is measured in the desired-to-undesired signal ratio (D/U). As an example, for an error rate better than 10e-09, QPSK requires a D/U of 16 dB. QAM-64 modulation, however, has a higher ideal spectral efficiency of 6 bits per hertz, but is more demanding, requiring a D/U ratio of 29 dB. Higher levels of spectral efficiency have been achieved, but only at much higher component costs, and with much higher D/U requirements. Most of the current digital modulation schemes were conceived over twenty years ago, and attention has turned to other methods of achieving spectral efficiency.
Digital compression involves using mathematical algorithms to reduce the amount of data sent, without losing any of the information. Compression is most effective with data that repeats certain patterns, such as video data. Raw data that exhibits no repetitive qualities benefits less from digital compression.
In addition to digital techniques, spatial methods can be used to achieve similar goals. Most UHF and microwave communications systems employ spatial techniques to increase the efficiency with which frequency spectrum is used over a given geographical area. Cellular systems exploit the limited range of these rf signals by reusing the same channels among multiple cell sites. The "Reuse factor" quantifies the efficiency of the particular "cellular reuse scheme." It is the distance between the centers of two cells that reuse the same channel divided by the radius of a cell (D/R). This number should be minimized.
Sectoring involves dividing the coverage area (cell) into pie-shaped slices, making possible increased levels of frequency reuse. Popular cellular reuse schemes also employ a small amount of sectoring. Generally, cells are divided into three sectors, as is evident from the triangular shape of cellular antenna systems. This allows more flexible allocation of available channels across the cellular system, and to a lesser degree, increased reuse.
Highly sectored antenna systems greatly increase the amount of reuse that can be achieved. As shown in FIG. 1, two or more of the sectors (slices) use the same frequency spectrum, achieving a "perfect" reuse factor of 1. The first quadrant of a 360.degree. coverage is shown sectored and sectors 29, 31 and 33 are depicted. Frequency 21 may be reused in another sector as shown; similar results can be achieved with frequency 23, 25 and 27, for example. Ideally, all of the sectors would use the same frequency spectrum, effectively multiplying the capacity of the spectrum by the number of sectors. For example, a 20 sector antenna system would lead to a 20-fold increase in capacity. But design issues affect the actual degree of sectoring that is achievable.
A sectored antenna system can consist of numerous discrete directional antennas colocated and aimed in different directions to establish a total 360 degree coverage. The aforementioned cellular systems use this method to divide cells into three sectors. However, highly sectored antenna systems are difficult to build and align using this method; there is a practical limit to the amount of sectoring that can occur. Also, such antenna systems are bulky and expensive due to the duplication of components.
A sectored antenna system is disclosed in U.S. patent application Ser. No. 08/677,413 for Focused Narrow Beam Communication System, incorporated herein by reference. This sectored antenna system utilizes one or more dielectric lenses. These lenses can be joined to create a hybrid lens device. In some cases, such a lens device may be analyzed and characterized as a single lens with unique properties. Such a lens is designed to have multiple focal points that serve as ports for the rf signals associated with each respective sector. Feed devices are mounted in close proximity to each desired focal point of the lens. The design of such feed devices is crucial to the performance of the sectored antenna system, and is a key element of the present invention. Microstrip or patch feed devices are used, though any appropriate feed devices may be employed.
Performance parameters for a sectored antenna system include gain, sidelobe and backlobe performance, and isolation among sectors. Feed device design affects all three of these parameters.
It is desirable to have high gain in the desired direction of each sector, with low sidelobe and backlobe levels to minimize the amount of radiation into other sectors. These objectives can be accomplished by increasing the size of the sectored antenna system, but it is desirable to keep the antenna system as small as possible. If such a sectored antenna system covers more than 90 degrees, it is likely that some feed devices will partially block the signals of other feeds, reducing the effective gain of those sectors of the antenna system. Such blockage must be reduced, but not at the cost of other design parameters.
Typically, lenses used in conjunction with a microstrip patch feed having low dielectric constant such as about 2.5 have been used and advantages of these appear in literature.